Method and composition for incorporating additives into molten metal

ABSTRACT

A METHOD OF, AND COMPOSITION FOR, INCORPORATING ADDITIVES INTO MOLTEN METALS, WHEREIN THE ADDITIVE IS BOUND OR CONTAINED IN A FIRST METAL OR ALLOY WHICH IS ADDED TO SAID MOLTEN METAL. FOR EXAMPLE, A HOMOGENEITY PROMOTER OF GRAPHITE AND A METAL CARBONATE CAN BE MIXED WITH SCRAP FILLINGS OF LEAD-BEARING ALLOYS AS A BINDER AND THE MIXTURE COMPACTED UNDER PRESSURE TO FORM A COHERENT PELLET WHICH CAN BE ADDED TO A MOLTEN ALLOY, SUCH AS A BRASS OR A BRONZE. AS THE METAL BINDER MELTS THE HOMOGENEITY PROMOTER WILL BE RELEASED.

3,713,813 METHOD AND COMPOSITION FOR INCORPORAT- ENG ADDHTIVES liNTO MOLTEN METAL (Jharles E. Lundin, Evergreen, Colo., assignor to Colorado Springs National Bank as trustee, Colorado Springs, Colo. No Drawing, Filed Oct. 8, 1970, Ser. No. 79,305

Int. Ci. C22c 9/08 US. Cl. 75-435 7 Claims ABSTRACT OF THE DISCLOSURE A method of, and composition IfOI, incorporating additives into molten metals, wherein the additive is bound or contained in a first metal or alloy which is added to said molten metal. For example, a homogeneity promoter of graphite and a metal carbonate can be mixed with scrap filings of lead-bearing alloys as a binder and the mixture compacted under pressure to form a coherent pellet which can be added to a molten alloy, such as a brass or a bronze. As the metal binder melts the homogeneity promoter will be released.

BACKGROUND Many attempts have been made to produce substantially homogeneous lead-containing alloys which are highly desirable for use in bearings. However, several problems have not been solved by the prior art in attempting to make such homogeneous lead-containing alloys. A basic problem is the segregation of the lead in alloys having from about up to about 50 percent lead in combination with other metals such as copper.

In patent application Ser. No. 705,640, filed Feb. 19, 1968, now US. Pat. 3,556,779, issued Jan. 19, 1971, by Robert Turkisher and Charles E. Lundin there is a disclosure of a homogeneity promoter for alloys comprising lead and copper. One example of said homogeneity promoter, or additive, is a mixture of graphite and sodium carbonate. The additive is incorporated into the molten metal, which may be at the temperature of about 1275 C. Thereupon violent agitation of the molten metal ensues with the formation of gas.

Although the exact mechanism is not completely understood, it is believed that the homogeneity promoter is partially decomposed to form gases which provide a stirring and nucleation effect and that the undecomposed portion of the promoter also provides nucleation sites. The carbonate melts well below the reaction temperature range and decomposes at the higher temperatures into carbon monoxide gas and the metal oxide. The oxide in turn is reduced by the carbon to form additional carbon monoxide and metal. The metal may also be above its boiling point and released to gaseous form. The combined action of the gases cause the vigorous stirring action. Agitation continues through the cooling step and it is in this stage when the agitation is believed to be most effective. The agitation prevents gross separation of the lead and the copper phases and durther provides many more nucleation sites for the solid, copperrich dendrites to form from the liquid. The additional nucleation sites cause the final solidified structure to be fine-grained and further allows more efficient and homogeneous entrapment and entrainment of the lead phase in the copper matrix. Also, inoculation occurs by unreacted (or partially reacted) carbonate and graphite during stirring of the melt. The inoculation by these particles which are not fully decomposed to gases provides sites for the nucleation and growth of fine lead particles. The combined action promotes a random, fine-grained dispersion of lead in copper which is mandatory for the optimum charnited States Patent O Hce acteristics and requirements of a bearing alloy. An additional benefit of the emission of carbon monoxide, or gaseous metal, is to produce a reducing atmosphere over the alloy during the liquification and solidification which reduces oxidation of the alloy from the air.

In practice, .molten baths of copper and lead prepared for casting purposes form a surface layer of a slag. The addition of a powdered mixture of graphite and sodium carbonate to such a melt is rendered less effective by the failure of the additive to penetrate through the molten metal. Even when such a slag is not present the addition of the additive, on immediate contact with the molten metal surface, causes a violent reaction at the surface with lessened beneficial effect to the entire melt. A further problem is that in practice the metal is maintained in a molten state for periods of from 3 minutes to one or more hours. However, the graphite and sodium carbonate mixture is of greatest advantage when the molten metal is at a high temperature tior only a period of several minutes.

In order to improve the dispersion of the additive throughout the molten metal, it is possible to add the graphite and sodium carbonate to an empty ladle or other container and then to pour the molten metal thereover. However, certain problems remain even with this method of addition. For example, the molten metal is often maintained at a high temperature for extended periods, during which the additive may be attenuated in its effect on the metal. Furthermore, it is of advantage to be able to add additional quantities of the graphite and sodium carbonate in successive increments to a bath of molten metal and to remelts of alloys.

Similar problems exist with other homogeneity promoters, including those disclosed in patent application Ser. No. 53,953 filed by said Turkisher and Lundin on July 10, 1970 as a continuation-in-part of Ser. No. 706,640, now US. Pat. 3,556,779, issued Ian. 19, 1971, and in patent application Ser. No. 62,338 on Aug. 10, 1970, filed by Lundin.

It is therefore an object of this invention to provide an improved method of, and composition for, incorporating additives into molten metals.

THE INVENTION This invention is directed to incorporating an additive into a metal binder or container to provide pellets which can be added to molten metal. Such pellets enable the user to incorporate the additive into the molten metal readily and in successive intervals over a period of time. The metal hinder or container for the additive should be more dense than, or of similar composition to the molten metal or one or more of the ingredients therein, to which it is added. This enables the pellet to sink to the bottom of a bath of molten metal whereupon the binder or container melts and releases the additive which is dispersed throughout the bath over a period of time. The hinder or container should, of course, be made of a metal which is compatible with the alloy in which it is placed and should have a melting point below that or in the range at which the alloy is melted for casting or other purposes.

The metal binder need not have a melting point below that of the alloy to which it is added, since it is often of advantage to superheat an alloy prior to casting it. The use of a high melting point metal binder for the additive can serve as an indicator, upon the release of the additive, that the alloy has been brought to a suificiently high temperature, and such a binder prevents premature release of the additive into the alloy at a time substantially before the alloy is ready to be poured or cast. For example, copper and manganese are among the higher melting point metals which can be used in binders or containers for homogeneity promoters in lead-based alloys. However, special care must be taken to prevent the pellets from floating on the top of the alloy to which they are added. The lead-based alloys themselves are of particular value as binders.

The use of pellets comprising additives in metal binders is also of value in the mixing of elemental metals. The pellet may be incorporated in one of the elemental metals in a molten state which is subsequently added to, or to which is added another elemental metal. By appropriate selection of the metal binder the time or temperature at which the additive is released may be controlled. In mixing elemental copper and lead, a copper binder or container having the additive may be added to the lead or molten lead. The additive will not be released until the lead is brought to the melting point of copper or until it is added to a bath of molten copper.

The invention of this application has particular utility in preparing a brass or bronze. or other alloy having a significant proportion of lead and/or copper. For such all ms the metal binder is preferably made of lead, copper, tin or one of the other constituents of the alloy, and combinations thereof. A readily available binder can be made from the scrap filings, cuttings or powders which are formed during the casting or machining of such alloys. These filings may be mixed with the additive and then readily compacted into a coherent mass with conventional equipment available in most foundries.

This invention is of particular value in producing copper-lead alloys having varying proportions of copper and lead. The proportions may be varied as desired and as the specific application dictates. It has been found that alloys of substantial utility are those which contain to 55% lead and 95% to 45% copper. The problem of separation and segregation of the lead and copper is greatest with a high lead content. The manner of adding promoter therefore provides its greatest utility at lead contents of from to The additive used in this invention may be any additive which is conventionally incorporated in alloys. However, significant advantages are obtained by the use of homogeneity promoters comprising elemental carbon, such as graphite, which may be used in combination with a metal compound wherein the metal is an alkali metal, alkaline earth metal, or rare earth metal in the form of an oxide or carbonate.

As to the homogeneity promoter used in this invention, it has been found that the elemental carbon component is preferably finely powdered graphite. Although coarser carbon may be used, the larger particles tend to decrease the efficiency of the process, presumably due to reduced surface area to volume ratio. Other forms of carbon include bone-black, carbon-black, charcoal and the like.

The alkali metal compound may be lithium, potassium or sodium (or other metals of Group la of the Periodic Table), preferably combined as a carbonate. The alkaline earth compound may be calcium, strontium or barium (or other metal of Group Ila of the Periodic Table) preferably combined as a carbonate. Combinations of alkali and alkaline earth compounds may be used, for example, a mixture of sodium and calcium carbonate. The terms rare earths and rare earth carbonates as used throughout this application are intended to include scandium, yttrium, lanthanum, and the lanthanides, the latter term encompassing those metals having atomic numbers from 58 to 71. The preferred rare earths are cerium and yttrium and mixtures thereof with lanthanum, praseodymium, samarium and europium. The preferred rare earth compounds are the halocarbonates, particularly the fluorocarbonates of the above metals. The carbonates and oxides of the rare earths are also suitable in this invention. The rare earth compounds may be used with or in place of the alkali and/or alkaline earth compounds. The amount of homogeneity promoter used must be at least that amount which ensures formation of a uniformly dispersed mixture of lead and copper which does not segregate on solidification. A preferred effective range of the proportions has been found to be about 15 grams of carbon or graphite powder and about 3-15 grams of metal compound for each pound of alloy. Below this proportion, improvements in homogeneity are obtained, but the effect is less pronounced when a very minor amount of promoter is used. Higher amounts of the homogeneity promoter may be used, for example, up to 10 grams of graphite and 30 grams of the metal compound for each pound of alloy. Although these and even greater amounts provide an improved alloy the use of greater amounts from an economic standpoint is less attractive. The maximum proportion of the promoter is determined by characteristic requirements of the alloy, and economic considerations.

Additional additives may be used in combination with the above-described homogeneity promoter. For example, from 1 to 10 grams of a metal phosphate may be used, such as ortho lead phosphate, ortho cupric phos. phate or ortho tin phosphate.

The amount of additive incorporated in the metal binder or container may vary widely. For example, for each part of additive, by volume, from one to twenty or more parts of metal may be used. Enough metal binder should be used to form a sufficiently strong pellet which can be handled under ordinary foundry conditions.

The amount of additive incorporated into the alloy is, in part, a function of the period during which the alloy is maintained in a molten state. If the alloy is to remain molten for several minutes, or more, it is desirable to add greater amounts of additive, either initially, or, preferably over a period of time. For example, the amounts of additive referred to above in a metal binder may be added to the molten metal every 3 to 10 minutes during which the molten state is maintained.

In incorporating the additive in its metal binder or container, it is important to ensure that no air or moisture is trapped therein. The presence of air or moisture is detrimental to most alloys and their entrapment within a hinder or container placed in molten metal may result in an explosion of the bound metal or container. For this reason it is preferred to mix scrap filings or powders with the additive and to compress the mixture under a high pressure. This procedure results in a coherent pellet which does not contain any air pockets or voids, and may be readily handled without breakage. Hydraulic presses for forming pellets from scrap metal are well known; these machines exert sufficient pressure on the metal to cause it to flow or form a dense and strong mass.

As the powdered compact pellet melts in the molten metal bath it starts to release its additive and continues to release the additive until the pellet is completely melted. The rate of release of the additive can be readily controlled by appropriate choice of the metal binder, the amount of additive in the binder and the temperature of the molten metal bath. Furthermore, the choice of the additive also affects the time interval during which the beneficial effects of the additive are released in the molten metal. For example, the additive may comprise a mixture of an alkali metal carbonate, such as sodium carbonate, and an alkaline earth metal, such as calcium carbonate, wherein the former decomposes before the latter.

A further method for the timed release of the additive is to incorporate it into two or more metal binders or foils which have different melting points. A portion of the additive may be mixed or bound with copper and compacted to form a pellet; further additive may be mixed or added to this pellet with a hinder or covering of lead which would envelop the first formed pellet. The relatively low melting point of lead would result in a rapid release of additive initially, whereas the higher melting point of copper would result in a subsequent release of further additive.

in place of the particulate form of metal binder exemplified above, i.e., filings, cuttings and powder, a metal foil or tube can be used to bind additives. The additive is placed into the foil or tube and the combination is compected into a pellet under elevated pressure to form a compact mass substantially free of voids. The use of layered foils or tubes of metals having different melting points is of particular value in controlling the timed release of the additive.

This invention will be illustrated in greater detail by reference to the following embodiments:

Example I A powdered mixture was prepared from 7 /2 lbs. of graphite and 16 /2 lbs. of sodium carbonate. The mixture was further mixed with 150 lbs. of metal filings of an alloy comprising approximately 20% lead, 4% zinc, 75% copper and minor amounts of antimony, nickel and other constituents. Portions of the mixture were then compacted under an elevated pressure into a coherent mass or pellet in a conventional hydraulic press used for compacting scrap metal. Each pellet weighed from 4 to 20 lbs. The pellets were then added to a bath of molten metal, of approximately the same composition as the metal filings; the bath was at about 23002400 F. A sufiicient number of pellets was added to incorporate approximately 7 lbs. of the additive mixture of graphite and sodium carbonate into a bath of approximately 350 to 400 lbs. of molten metal. The pellets gravitated downward through the molten metal and the release of the additive was noted by ebullition throughout the molten mass.

The molten metal was then centrifugally cast by conventional procedures to form a motor support bearing. The casting proceeded smoothly and the molten metal flowed very well under the centrifugal pressure. Upon cooling, the bearing was broken in a press and the internal structure visually examined. It was observed that excellent dispersion of the constituents of the alloy was obtained.

Example II Additional pellets were made by the same technique from the above-mentioned mixture of graphite and sodium carbonate. However, in this example 35 lbs. of the additive mixture were incorporated in 150 lbs. of filings of an alloy comprising 15% lead, 80% copper, 4% zinc, and trace amounts of nickel, antimony and other constituents. These pellets, used in the manner noted above, produced excellent alloys suitable for bearings.

Further formulations of homogeneity promoters suitable for use in the present invention include the following.

Calcium carbonate, 136 grams, and powdered graphite, 34 grams, may be compacted in a metal binder and added to a melt of 6 pounds of lead and 9 pounds of copper.

A commercially available rare earth mixture, 123 grams, and powdered graphite, 34 grams, may be compacted in a metal binder and added to a melt of 6 pounds of lead and 9 pounds of copper.

The rare earth mixture is known as bleached rare earth fluorocarbonates, containing predominantly the cerium compound, with minor amounts of lanthanum, praseodymium and neodymium fluorocarbonates and trace amounts of other rare earth fluorocarbonates, such as samarium and europium.

Further formulations of homogeneity promotors are set forth in the patent applications of Lundin, and Turkisher and Lundin, referred to above.

This invention has been described in terms of specific embodiments set forth in detail. Alternative embodiments will be apparent to those skilled in the art in view of this disclosure, and accordingly such modifications are to be contemplated within the spirit of the invention as dis closed and claimed herein.

I claim:

1. The method of incorporating an additive into a bath of molten metal which comprises mixing said additive with a particulate metal binder, compacting said additive and metal binder under elevated pressure to produce a coherent mass and then adding said coherent mass to the molten metal, wherein said coherent mass is sufficiently dense to sink within said molten metal and said metal binder is compatible with and melts within said molten metal to release said additive.

2. The method of claim 1 wherein said metal binder comprises particles of a lead-containing alloy, and said molten metal comprises copper and lead.

3. The method of claim 1 wherein said pressure is sufficiently high to produce a substantially void-free mass.

4. The method of claim 3 wherein the additive in a metal binder is added to the molten metal in successive increments.

5. The method of claim 2 wherein said additive comprises elemental carbon and a metal compound that reacts therewith at the elevated temperatures of said molten metal, said metal compound is selected from the group consisting of compounds of an alkali metal, an alkaline earth metal and rare earth metal, and about 1 to 5 grams of said carbon and 3 to 15 grams of said metal compound are added for each pound of molten metal.

6. The method of claim 1 wherein said pressure causes said metal binder to flow.

7. The method of incorporating an additive into a bath of molten metal which comprises mixing said additive with a metal binder, compacting said additive and metal binder under elevated pressure to produce a coherent mass and then adding said coherent mass to the molten metal, wherein said additive comprises finely powdered elemental carbon and a metal compound that reacts therewith at the elevated temperatures of said molten metal, said metal compound is selected from the group consisting of compounds of an alkali metal, an alkaline earth metal and rare earth metal, said metal binder comprises lead, and said molten metal comprises copper and lead.

References Cited UNITED STATES PATENTS 2,343,761 3/1944 Fleming et al -94 X 3,052,535 9/1962 Peters 7594 X 3,556,779 1/ 1971 Turkisher et al. 75163 X L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R. 

